Sensor having differential polarization and a network comprised of several such sensors

ABSTRACT

Parametric processing capability is added to a typical sensor so that a target object can be more clearly distinguished from the background clutter in a given scenery. A polarizer with several segments of different polarization orientations is used to improve the typical sensor. The segments are sequentially advanced to pass therethrough infrared radiation images of pre-selected polarization orientations which are then collected by respective polarized frame grabbers. Image processing circuit processes these images to yield the polarization difference between any given pair of orthogonal polarizations. In a surveillance network, the polarization differences are subsequently used in the control center, to which such sensors are connected, to enhance the distinction of the observed objects against the background clutter suspended in the propagation medium.

DEDICATORY CLAUSE

The invention described herein may be manufactured, used and licensed byor for the Government for governmental purposes without the payment tome of any royalties thereon.

BACKGROUND OF THE INVENTION

In these times of heightened security, there are many instances whenparticular geographic areas may need to be placed under surveillance toprotect potential targets from terrorist attacks or as an adjunct tomilitary operations.

When an object is imbedded in a medium such as rain or fog, the presenceof scatterers in the medium causes the image of the object to appearindistinct or blurred, making object detection difficult. One aspect ofman-made objects that may assist in their detection, however, is thatthey emit and reflect near-infrared radiation that is more highlypolarized than does natural background that may include trees, brushgrass or terrain.

There are two broad categories of infrared systems designed for militaryuse; namely, scanning system and staring system. However, infraredsensors for extant missile seekers and forward-looking infrared (FLIR)sensors lack the capability for detecting the polarization orientationof the incident radiation, i.e. they respond to any polarizationorientation vector of the incident radiation. Studies have shown thatthe capability to detect and analyze the polarization orientation ofnear-infrared radiation emanating or reflecting from targets andbackground scenery can provide a potential means for improving thedetection and discrimination of the targets in military systems.Infrared instrumentation having this feature would also have potentialcommercial applications in areas such as pattern recognition, materialscharacterization and analysis.

Thus, one way to improve the performance of target detection system isto utilize polarized infrared signatures of a given scenery. Whatrenders this possible is that polarization properties are independent oftarget-background contrast (i.e. polarization properties are distincteven when there is no temperature difference between the target and thebackground) and of normal infrared target and clutter temperaturefluctuations (i.e. standard deviation of temperature). As is well-known,the formal characterization of an electromagnetic wave that may belinear, elliptical or circular is accomplished with the derivation ofthe four Stokes parameters.

SUMMARY OF THE INVENTION

This invention relates to a system for detecting and processing passiveand active polarized near-infrared radiation for applications in devicessuch as research, instrumentation, surveillance systems, infraredmissile sensors or seekers, search and acquisition systems and guidancesystems.

Polarimetric processing capability is added to a typical sensor so thata target object can be more clearly distinguished from the backgroundclutter in a given scenery. An embodiment to accomplish this involvesusing a polarizer with several segments of different polarizationorientations in conjunction with the microchannel image intensifier tubeas taught by David B. Johnson, et al in U.S. Pat. No. 5,373,320 (Dec.13, 1994), resulting in improved sensors. The polarization segments aresequentially advanced to pass therethrough infrared radiation ofpre-selected polarization orientations, which radiation, then, impingeson the charge coupled device (CCD) camera. Multiple polarized framegrabbers coupled to the camera produce image frames of the respectivepre-selected polarization orientations and image processing circuit thenprocesses these image frames to yield the polarization differencebetween any given pair of orthogonal polarizations. The polarizationdifferences are subsequently used to enhance the distinction of theobjects against the background clutter suspended in the propagationpath.

In a surveillance system many such sensors placed to observe diversegeographical locations can be connected to a control center thatcontrols and coordinates the functions of the individual sensors. Also,the control center receives relevant polarization information from thesensors and correlates the information for a more effective surveillanceof the diverse locations.

DESCRIPTION OF THE DRAWING

FIG. 1 shows a diagram of a preferred embodiment of the improved sensor,the parts enclosed in the dashed lines indicating the improvement overthe Johnson system.

FIG. 2 illustrates the several polarization segments of the polarizer101.

FIG. 3 depicts a surveillance network comprising a plurality of thesensors.

FIG. 4 is a flowchart of a typical algorithm that may be used by controlcenter 300 to process the inputs from individual sensors 100.

DESCRIPTION OF THE PREFERRED EMBODIMENT

This invention is an improvement that may be used with the cameraattachment that converts a standard daylight video camera into aday/night-vision video camera as taught by Johnson in the U.S. Pat. No.5,373,320 (Dec. 13, 1994). Therefore, the disclosure of the Johnsonpatent is incorporated herein in its entirety.

Referring now to the drawing wherein like numbers represent like partsin each of the several figures, the improvement to impart differentialpolarization capability to sensor 100 is explained in detail. Any andall of the numerical dimensions and values that follow should be takenas nominal values rather than absolutes or as a limitation on the scopeof the invention.

In FIG. 1, near-infrared radiation 59 emanating or reflecting from theobserved scenery (not shown in the figure) enters sensor 100 throughiris lens assembly 60 which may be an auto iris lens. At a pre-selectedlow level of ambient illumination (usually night-time or a very darkday), image intensifier tube 61 having coupled thereto photocathode 117,and polarizer 101 swing into place to be in the path of the incomingradiation 59. The radiation then impinges on polarizer 101, a particularsegment of the polarizer having a specific polarization orientationbeing in the path of the incoming radiation. The particular segment isone of several segments of the polarizer, each segment having a distinctpolarization orientation and transmitting only the portion of theincoming radiation that has the same polarization orientation as that ofthe particular segment in the radiation path. As illustrated in FIG. 2,the several segments of polarizer 101 comprise four at 0°, 45°, 90°, and135° orientations, respectively. Actuator 103, powered by power source62, translates the polarizer segments sequentially in discrete stepsalong the length of image intensifier tube 61 so that they aresequentially engaged to be in the radiation path between iris lensassembly 60 and the image intensifier tube, which may be atwo-dimensional microchannel plate.

The radiation transmitted through the engaged polarizer segment is thenbrought to a focus on photocathode 117 that covers all the pixelelements of the microchannel plate (image intensifier tube). It is notedhere that the particular segment of the polarizer engaged at any givenmoment in time should be sufficiently large to cover all the pixelelements of the microchannel plate. For each pixel element of theradiated image, electrons are emitted from the rear of the photocathode.The electrons thusly emitted from each pixel position enter thecorresponding channels of the microchannel plate and are multiplied inthe channels of the microchannel plate, subsequently exiting as outputlight from the microchannel plate and being relayed by the relay lens 63to be incident on CCD camera 64.

A plurality of polarized frame grabbers 105, 107, 109 and 111, each witha different polarization orientation, are selectively connected to theCCD camera via first switch 119 and second switch 121. The operation ofthe switches is synchronized with the operation of actuator 103 suchthat the polarization orientation of the engaged polarizer segment isthe same as that of the connected polarizer frame grabber. In exemplaroperation of the above-described process, the image at 0° is read intoframe grabber 105; then the actuator translates the polarizer to engagethe 90°-segment, resulting in the image at 90° being read into framegrabber 107. Then the logic circuitry at the output of the two polarizedframe grabbers forms both the sum, V₀+V₉₀, and the difference, V₀+V₉₀,of the two orthogonal polarizations. From this, the normalized ratio isformed:

${{Polarization}\mspace{14mu}{Difference}} = \frac{V_{0} - V_{90}}{V_{0} + V_{90}}$This information is transmitted to control center 300 for furtherprocessing as will be explained below. Collecting two orthogonalpolarizations of an object image and differentiating between the two asillustrated above improves the detectability of the object. This processis known as polarization difference imaging and its general principlesare explained in U.S. Pat. No. 5,975,702 (Nov. 2, 1999). The process ofsummation and differencing and obtaining the ratio can be performed withany other pair of orthogonal polarizations.

Since the polarizer functions with the microchannel plate that amplifiesthe low light level, a condition that is prevalent in night-time, thepolarizer is not operational during daylight operation of the sensor,but is removed from the path of the incoming radiation. Thus, at aselected higher level of ambient illumination, under the control ofcontrol center 300 the image intensifier tube (microchannel plate) andthe polarizer are rotated out of position and, in their place, opticalpath length compensator 67 is rotated into position and second switch121 connects the CCD camera with unpolarized frame grabber 125. With thecompensator in place, unpolarized imagery may now be collected by theunpolarized frame grabber and further processed by the control center.

A plurality of ground sensors 100 (perhaps placed along a defensiveperimeter to detect hostile intrusion by man or vehicles) may beconnected via wireless links or fiber optic links 301 to control center300 as depicted in FIG. 3 to form a surveillance network. Additionalsensors (having the same or different capabilities as the groundsensors) on air-borne platforms may be connected to the control centeras well for greater versatility of the network. In such a network, thecontrol center performs correlation of the imagery inputs from theground sensors with each other as well as with imagery inputs fromair-borne sensors and other sources, such as satellites, to identify andtrack a target object against background clutter. For example, thesatellite or air-borne sensors, having the capability to view a largearea from their elevated positions, can alert the ground sensors tohostile movements toward the protected perimeter. The ground sensors, onthe other hand, having visibility underneath vegetation canopy, may beable to alert the elevated sensors to movements hidden from their view.In addition to transmitting data from the sensors to the control center,links 301 also allows the control center to adjust the settings at eachof the sensors: for example, the frame readout rates of each sensor, theselective connections of switches 119 and 121, the occasional activationof power source 62 and the level of incoming radiation through iris lensassembly 60.

It is assumed that the control center is under the supervision of anexpert image analyst, as indicated in FIG. 4, who is familiar with thetopography and understands the physics of the diurnal and long termchanges in the scenes observed, even though the image processingoperations may be performed autonomously or semi-autonomously. Theterrain features being observed by the sensors in the surveillancesystem may include natural and man-made features both of which changeover time. The scenes being observed may include roads (a target areafor emplacement of roadside bombs), building structures that may concealsnipers, a forest area that might conceal military movement or grassyarea where land mines may be buried. In all these applications, thescene images are obtained at different points in time that may extendover a period of an hour to a day, or a period of a day to weeks or tomonths. This requires that a signal model of each of the emplacedsensors be developed so that variations can be identified, andimperfections in time be obtained to compensate for those variations ineach sensor. A suitable calibration procedure is described by MehrdadSoumekh et al in “Time Series Processing of FLIR Imagery for MTI andChange Detection” in the IEEE's Proceedings of the InternationalConference on Acoustics, Speech and Signal Processing, Vol. 5, pages21–24 (2003). The calibration procedure is performed under thesupervision of the control center and its result becomes a part of thesite model as depicted in FIG. 4.

Initially, to build the site model, an aerial image of a site (a givenscene) is obtained as a reference. Then subsequent images of the sitefrom emplaced ground sensors or air-borne sensors are used to update andverify the site model, utilizing the principle that when two or moreimages of a given scene are available and the geometric relationshipbetween the images and the geo-spatial coordinates are known with somedegree of accuracy, the collected images can be coordinated. FIG. 4illustrates a representative software architecture for implementingsignal processing operation between the multiple images obtained fromthe several ground emplaced sensors or aerial sensors. It is noted thatthe aerial sensors and the ground sensors may not be of the same type;it is envisioned that the aerial sensors would include all-weathersensors, such as synthetic aperture radar (SAR), but the ground sensorswould not. The aerial sensors may also be hyperspectral sensors whoseseveral spectral bands can be directly coordinated with the groundsensors.

The main operation of control center 300 is achieving the match betweenimages from the same or different sensor types taken at different timesand different resolutions. A common approach is to designate one image,an aerial image for example, as a reference and perform a rotation orwarping of the second image to bring it into alignment with thereference image. Many algorithms are already known in the art that canbe used to coordinate multiple images, depending on the nature of theparticular images. For example, when comparing infrared images atwavelengths shorter than 4–5 microns and those at longer infraredwavelengths, a suitable algorithm used to achieve a match between theday-time images and night-time images is the Multivariate MutualInformation Algorithm as is explained in “Registration of Image CubesUsing Multivariate Mutual Information,” by Jeffrey P. Kern et al inIEEE's Thirty-Seventh ASILOMAR Conference: Signals, Systems andComputers, Vol. 2, pages 1645–1649 (2003). Another example is when theair-borne sensor includes a SAR. In such a case, the fusion of the SARimage with near-infrared image will be the required signal processing.Yet a third example is when the targets are far from the sensors andthus appear in the image as points against the background clutter. Inthis case, the necessary signal processing may include a multi-scaledecomposition based on wavelets that allows the detection andcharacterization of the dynamic elements in the scene. An algorithm toaccomplish such decomposition is explained in “Multiple Single Pixel DimTarget Detection in Infrared Image Sequence,” by Mukesh A. Zaveri et alin IEEE's Proceedings of International Conference on Circuits andSystems, Vol. 2, pages II-380 to II-383 (2000).

Although a particular embodiment and form of this invention has beenillustrated, it is apparent that various modifications and embodimentsof the invention may be made by those skilled in the art withoutdeparting from the scope and spirit of the foregoing disclosure. Forexample, to obtain two frames of orthogonal polarizations at the sameinstant in a given sensor, one can use a stereo camera with the twoorthogonal polarizers fixed to the two separate sets of optics so thetwo frames of orthogonal polarizations are obtained at the same instant.This may avoid the image smearing that can happen if motion occursduring the transition of the polarizer from one polarization segment toanother but would correspondingly require additional intensifier, relaylens and switching mechanism, rendering the system bulkier and morecomplicated. Another modification is to use sensors that are capable ofswitching between daylight and night vision operational modes. Toprovide differential polarization during daylight operations, a secondpolarizer-actuator assembly would need to be integrated with the opticalelements of the sensor. Accordingly, the scope of the invention shouldbe limited only by the claims appended hereto.

1. In a system for viewing a pre-selected scenery in the near-infraredspectrum, the system having an iris lens assembly for collectingincoming radiation, a charge-coupled device camera; a relay lens forfocusing the collected radiation onto the CCD camera; a microchannelimage intensifier positioned between the iris lens assembly and therelay lens, an optical path length compensator selectively-placeablebetween the iris lens and the image intensifier, the intensifieramplifying the incoming radiation level prior to transmitting theradiation to the relay lens; and a photocathode coupled to the imageintensifier, AN IMPROVEMENT for providing a polarimetric processingcapability to the system so as to enable distinction of various featuresof the pre-selected scenery, said IMPROVEMENT comprising: a polarizerplaceable between the iris lens and the photocathode, said polarizerbeing composed of several discrete segments of different polarizationorientations and being translatable along the length of the imageintensifier in discrete steps so as to bring sequentially said discretepolarization segments to be engaged between the iris lens and thephotocathode, each of said segments being of sufficient size to coverall the pixels of the image intensifier; a plurality of polarized framegrabbers coupled to the CCD camera, each polarized frame grabberreceiving radiation of a particular polarization orientation andoutputting an image frame of that particular polarization orientation;and a logic circuit for performing image processing on said image frameoutputs from said polarized frame grabbers and producing therefrompolarization difference of any given pair of orthogonal polarizationorientations, said polarization of difference being used to enhance thevisual perception of the diverse features of the scenery.
 2. AnIMPROVEMENT as set forth in claim 1, wherein said improvement furthercomprises: a means to translate said polarizer in said sequential manneralong the length of the image intensifier.
 3. An IMPROVEMENT as setforth in claim 2, wherein each of said several discrete segments of saidpolarizer covers all of the pixel elements in the microchannel imageintensifier.
 4. An IMPROVEMENT as set forth in claim 3, wherein saidimprovement still further comprises: a first switch coupled between theCCD camera and said polarized frame grabbers, said first switchselectively connecting, from time to time, said CCD camera with one ofsaid polarized frame grabbers.
 5. An IMPROVEMENT as set forth in claim4, wherein said first switch is selectively connected to said polarizedframe grabbers in synchronization with said translating means such that,at any given moment in time, said engaged polarization segment and saidconnected polarized frame grabber are of the same polarizationorientation.